Look on side of a psu. Incoming AC current Volt 120V with Amperage = total AC current withdrawn. But Let's say your 500W psu draw only 300w from computer will it still need the same AC current input or it's auto adjusting also like the Dc side of the psu that power the computer?

Its like this: A 500 W PSU will, at max, output 500 W. It will as JR said, adjust to the demands of whatever it is powering on the intake side as it will only draw what is needed up to 500 watts. If it is 80% effecient this means to give that 500 W it will be drawing 625 watts (120 VAC x 5.208 amps).

Ok thank you! I would like to use a 1000W Dell psu someone gave me and use it to power up the computer and other 12v stuff like light bulbs and fans for my bedroom. Didn't want it to draw a shit load of current to light a single DEL bulb

Its like this: A 500 W PSU will, at max, output 500 W. It will as JR said, adjust to the demands of whatever it is powering on the intake side as it will only draw what is needed up to 500 watts. If it is 80% effecient this means to give that 500 W it will be drawing 625 watts (120 VAC x 5.208 amps).

A 500w PSU, will will use a maximum of 500w (normally) inside the computer. like TheLaughingMan said, if it has a effiency of 80% it will draw 625w at the Wall, because of the Joule Effect. When electricity go into the power supply, there will be heat, and you will loose a bit of power. so if you draw 625 from the wall, you PSU will delivrer 500w inside the computer, if that PSU has an effiency of 80% (Which means there is a lost of 20% from the joule effect)

Big example, if you plug your christmas tree, you won't take 1200w, it will take what it needs. I'm not really sure of your question, but I hope this answer. If you don't use all the power available, your PSU won't use it, it will use what it need, and what he is able to use.

Ok it all answer my questions. So I'll figure out how to go on with my 12V setup project. I know for sure I could cut some 12V wires on it and put on a car lighter adapter to charge many things at same time like cell phone, portable dvd player, etc.

2, if good make
a, If PC need idle, lets assume 150 watt usage , then your PSU may pull 200watt or more from wall.
b, you play on PC , lets say PC need 300 watt , then your PSU will draw from wall 360 watt
( god make usually write what the PSU CAN deliver to your PC )